1. Field of the Invention
The present invention relates to a process of stretching tubular nets or plates made of extruded plastic material coming out of an extruder or extrusion head, according to one or two directions at right angles to each other and to the equipment for carrying out the same.
The process and the equipment in question are particularly used in the field of the processings applied to plastic materials capable of being deformed and extended where, starting from the step in which the material is extruded in the form of a continuous plate, or sheet it is possible to obtain a stretched finished product which can take the form of a solid even plate, a perforated plate provided with differently disposed and shaped holes or even of a real net.
2. Prior art
At present, in order to obtain the stretching of plates or nets made of extruded plastic material according to two directions at right angles to each other, very complicated equipments are used that carry out the longitudinal and transverse stretch operations in two steps and in two succeeding stations.
One of these equipments is disclosed in U.S. Pat. No. 4,152,479 issued on May 1st, 1979 which you can refer to for more details.
Substantially the apparatus shown therein comprises a plurality of stretching rollers disposed one after the other with the respective axes parallel to each other. The material coming out of the extrusion head, either in the form of a net or of a plate, is caused to pass through said rollers which are rotated at differentiated rates. In order to make the stretch operation easier, rollers are suitably heated so that they can transmit heat by conduction to the product coming in contact with them.
Owing to the differentiated rates of said rollers, longitudinal stresses take place in the pre-heated product which cause the product to be stretched in this sense. Once the product has been longitudinally stretched, the net or plate is fed to a further equipment comprising two chains of pliers or grippers disposed along the sides of the net or plate. The chains follow a path that first extends parallelly to the axis of the net or plate, then opens wide apart therefrom and finally runs parallelly to said axis again. In this way pliers or grippers can grasp the net or plate coming from the set of rollers at the edges thereof and draw it transversely along the path until it reaches the desired width.
In order to assist the stretch operation in a transverse direction on the net or plate, hot air is continuously blown in.
A takeup pulley disposed downstream of the pliers chains is provided in order to limit the longitudinal shrinkage of the net or plate which inevitably occurs during the transverse stretch operation.
Although equipments of the kind described above have been widely used in stretching nets or plates, they have many drawbacks and limitations both from a technical and operational standpoint and from a pratical one.
A first drawback that can easily be found is given by the fact that these equipments can carry out the longitudinal and transverse stretch operations only on nets or plates of a reduced thickness. In fact, referring above all to nets, the metal rollers used in the longitudinal stretch operation are capable of transmitting only a reduced amount of heat to the wires constituting the nets, as the contact surfaces between the rollers and the wires appear very small with respect to the whole surface of the wires. Finally, wires can be heated in a suitable manner only if they have a very thin cross-section; in fact, should their section be thicker, they would be heated on the surface only and, by effect of the drawing stresses, they would break. The same remarks are also valid when the net passes through the transverse stretch station, as the heating means, that is the air blown in, has a substantially low specific heat which allows a suitable heating of the net only if it is made from thin wires.
Therefore, by using the equipments described above it is not possible to carry out a stretch operation on the nets when the wires constituting the same have a thick section, due to the fact that the heating means (a mere contact between a metal surface and the wire or the blowing in of hot air) appears inadequate.
A further drawback linked to the above equipments is given by the presence of pliers grasping the edges of a net or a plate during the transverse stretch step. It should be noted first that the grasp of said pliers is discontinuous and that no kind of stretching can be executed on the area caught by the pliers. As a result, the edges of the net appear more compact. As a consequence of that, when the finished product is wound around a reception bobbin, the winding of the side edges is more tensioned, which can cause the net to be broken just along the edges thereof. Furthermore, this kind of break can also take place rather often during the transverse stretch step when the pliers, being subjected to a sudden straddle, exert a transverse jerking action on the net.
A further drawback linked to the prior art described above is given by the fact that these equipments have big sizes and take up too much room inside a factory. Usually the plan bulkness of these equipments takes up many tens of square meters.
A still further drawback, linked to the stretch technique carried out by two succeeding longitudinal and transverse tensioning steps of the net is given by the fact that, on their junction points, the wires are considerably stretched which gives rise to the formation of rather flattened knots. For this reason, when nets are used and there is the occurrence of stresses that are not in the same direction as the wires but in a bias direction, owing to the very reduced possibility of deformation of plastic material at room temperature, the knots tend to crack which causes the net to be broken.
A still further drawback linked to the inherent structure of the machines presently used is given by the fact that when nets of different widths have to be obtained, it is always necessary to stop the equipments and carry out a series of adjustments, which requires time and labour and consequently adversely affects the cost of the finished product.